1. Field of the Invention
The invention pertains to the field of variable camshaft timing (VCT) systems. More particularly, the invention pertains to method and apparatus to reduce noise of a cam Phaser by controlling the position of center mounted spool valve.
2. Description of Related Art
The performance of an internal combustion engine can be improved by the use of dual camshafts, one to operate the intake valves of the various cylinders of the engine and the other to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both of the camshafts can be driven by a single crankshaft powered chain drive or belt drive. Engine performance in an engine with dual camshafts can be further improved, in terms of idle quality, fuel economy, reduced emissions or increased torque, by changing the positional relationship of one of the camshafts, usually the camshaft which operates the intake valves of the engine, relative to the other camshaft and relative to the crankshaft, to thereby vary the timing of the engine in terms of the operation of intake valves relative to its exhaust valves or in terms of the operation of its valves relative to the position of the crankshaft.
Consideration of information disclosed by the following U.S. patents, which are all hereby incorporated by reference, is useful when exploring the background of the present invention.
U.S. Pat. No. 5,002,023 describes a VCT system within the field of the invention in which the system hydraulics includes a pair of oppositely acting hydraulic cylinders with appropriate hydraulic flow elements to selectively transfer hydraulic fluid from one of the cylinders to the other, or vice versa, to thereby advance or retard the circumferential position on of a camshaft relative to a crankshaft. The control system utilizes a control valve in which the exhaustion of hydraulic fluid from one or another of the oppositely acting cylinders is permitted by moving a spool within the valve one way or another from its centered or null position. The movement of the spool occurs in response to an increase or decrease in control hydraulic pressure, PC, on one end of the spool and the relationship between the hydraulic force on such end and an oppositely direct mechanical force on the other end which results from a compression spring that acts thereon.
U.S. Pat. No. 5,107,804 describes an alternate type of VCT system within the field of the invention in which the system hydraulics include a vane having lobes within an enclosed housing which replace the oppositely acting cylinders disclosed by the aforementioned U.S. Pat. No. 5,002,023. The vane is oscillatable with respect to the housing, with appropriate hydraulic flow elements to transfer hydraulic fluid within the housing from one side of a lobe to the other, or vice versa, to thereby oscillate the vane with respect to the housing in one direction or the other, an action which is effective to advance or retard the position of the camshaft relative to the crankshaft. The control system of this VCT system is identical to that divulged in U.S. Pat. No. 5,002,023, using the same type of spool valve responding to the same type of forces acting thereon.
U.S. Pat. Nos. 5,172,659 and 5,184,578 both address the problems of the aforementioned types of VCT systems created by the attempt to balance the hydraulic force exerted against one end of the spool and the mechanical force exerted against the other end. The improved control system disclosed in both U.S. Pat. Nos. 5,172,659 and 5,184,578 utilizes hydraulic force on both ends of the spool. The hydraulic force on one end results from the directly applied hydraulic fluid from the engine oil gallery at full hydraulic pressure, PS. The hydraulic force on the other end of the spool results from a hydraulic cylinder or other force multiplier which acts thereon in response to system hydraulic fluid at reduced pressure, PC, from a PWM solenoid. Because the force at each of the opposed ends of the spool is hydraulic in origin, based on the same hydraulic fluid, changes in pressure or viscosity of the hydraulic fluid will be self-negating, and will not affect the centered or null position of the spool.
U.S. Pat. No. 5,289,805 provides an improved VCT method which utilizes a hydraulic PWM spool position control and an advanced control algorithm that yields a prescribed set point tracking behavior with a high degree of robustness.
In U.S. Pat. No. 5,361,735, a camshaft has a vane secured to an end for non-oscillating rotation. The camshaft also carries a timing belt driven pulley which can rotate with the camshaft but which is oscillatable with respect to the camshaft. The vane has opposed lobes which are received in opposed recesses, respectively, of the pulley. The camshaft tends to change in reaction to torque pulses which it experiences during its normal operation and it is permitted to advance or retard by selectively blocking or permitting the flow of engine oil from the recesses by controlling the position of a spool within a valve body of a control valve in response to a signal from an engine control unit. The spool is urged in a given direction by rotary linear motion translating means which is rotated by an electric motor, preferably of the stepper motor type.
U.S. Pat. No. 5,497,738 shows a control system which eliminates the hydraulic force on one end of a spool resulting from directly applied hydraulic fluid from the engine oil gallery at full hydraulic pressure, PS, utilized by previous embodiments of the VCT system. The force on the other end of the vented spool results from an electromechanical actuator, preferably of the variable force solenoid type, which acts directly upon the vented spool in response to an electronic signal issued from an engine control unit (“ECU”) which monitors various engine parameters. The ECU receives signals from sensors corresponding to camshaft and crankshaft positions and utilizes this information to calculate a relative phase angle. A closed-loop feedback system which corrects for any phase angle error is preferably employed. The use of a variable force solenoid solves the problem of sluggish dynamic response. Such a device can be designed to be as fast as the mechanical response of the spool valve, and certainly much faster than the conventional (fully hydraulic) differential pressure control system. The faster response allows the use of increased closed-loop gain, making the system less sensitive to component tolerances and operating environment.
U.S. Pat. No. 5,657,725 shows a control system which utilizes engine oil pressure for actuation. The system includes a camshaft has a vane secured to an end thereof for non-oscillating rotation therewith. The camshaft also carries a housing which can rotate with the camshaft but which is oscillatable with the camshaft. The vane has opposed lobes which are received in opposed recesses, respectively, of the housing. The recesses have greater circumferential extent than the lobes to permit the vane and housing to oscillate with respect to one another, and thereby permit the camshaft to change in phase relative to a crankshaft. The camshaft tends to change direction in reaction to engine oil pressure and/or camshaft torque pulses which it experiences during its normal operation, and it is permitted to either advance or retard by selectively blocking or permitting the flow of engine oil through the return lines from the recesses by controlling the position of a spool within a spool valve body in response to a signal indicative of an engine operating condition from an engine control unit. The spool is selectively positioned by controlling hydraulic loads on its opposed end in response to a signal from an engine control unit. The vane can be biased to an extreme position to provide a counteractive force to a unidirectionally acting frictional torque experienced by the camshaft during rotation.
U.S. Pat. No. 6,247,434 shows a multi-position variable camshaft timing system actuated by engine oil. Within the system, a hub is secured to a camshaft for rotation synchronous with the camshaft, and a housing circumscribes the hub and is rotatable with the hub and the camshaft and is further oscillatable with respect to the hub and the camshaft within a predetermined angle of rotation. Driving vanes are radially disposed within the housing and cooperate with an external surface on the hub, while driven vanes are radially disposed in the hub and cooperate with an internal surface of the housing. A locking device, reactive to oil pressure, prevents relative motion between the housing and the hub. A controlling device controls the oscillation of the housing relative to the hub.
U.S. Pat. No. 6,250,265 shows a variable valve timing system with actuator locking for internal combustion engine. The system comprising a variable camshaft timing system comprising a camshaft with a vane secured to the camshaft for rotation with the camshaft but not for oscillation with respect to the camshaft. The vane has a circumferentially extending plurality of lobes projecting radially outwardly therefrom and is surrounded by an annular housing that has a corresponding plurality of recesses each of which receives one of the lobes and has a circumferential extent greater than the circumferential extent of the lobe received therein to permit oscillation of the housing relative to the vane and the camshaft while the housing rotates with the camshaft and the vane. Oscillation of the housing relative to the vane and the camshaft is actuated by pressurized engine oil in each of the recesses on opposed sides of the lobe therein, the oil pressure in such recess being preferably derived in part from a torque pulse in the camshaft as it rotates during its operation. An annular locking plate is positioned coaxially with the camshaft and the annular housing and is moveable relative to the annular housing along a longitudinal central axis of the camshaft between a first position, where the locking plate engages the annular housing to prevent its circumferential movement relative to the vane and a second position where circumferential movement of the annular housing relative to the vane is permitted. The locking plate is biased by a spring toward its first position and is urged away from its first position toward its second position by engine oil pressure, to which it is exposed by a passage leading through the camshaft, when engine oil pressure is sufficiently high to overcome the spring biasing force, which is the only time when it is desired to change the relative positions of the annular housing and the vane. The movement of the locking plate is controlled by an engine electronic control unit either through a closed loop control system or an open loop control system.
U.S. Pat. No. 6,263,846 shows a control valve strategy for vane-type variable camshaft timing system. The strategy involves an internal combustion engine that includes a camshaft and hub secured to the camshaft for rotation therewith, where a housing circumscribes the hub and is rotatable with the hub and the camshaft, and is further oscillatable with respect to the hub and camshaft. Driving vanes are radially inwardly disposed in the housing and cooperate with the hub, while driven vanes are radially outwardly disposed in the hub to cooperate with the housing and also circumferentially alternate with the driving vanes to define circumferentially alternating advance and retard chambers. A configuration for controlling the oscillation of the housing relative to the hub includes an electronic engine control unit, and an advancing control valve that is responsive to the electronic engine control unit and that regulates engine oil pressure to and from the advance chambers. A retarding control valve responsive to the electronic engine control unit regulates engine oil pressure to and from the retard chambers. An advancing passage communicates engine oil pressure between the advancing control valve and the advance chambers, while a retarding passage communicates engine oil pressure between the retarding control valve and the retard chambers.
U.S. Pat. No. 6,311,655 shows multi-position variable cam timing system having a vane-mounted locking-piston device. An internal combustion engine having a camshaft and variable camshaft timing system, wherein a rotor is secured to the camshaft and is rotatable but non-oscillatable with respect to the camshaft is discribed. A housing circumscribes the rotor, is rotatable with both the rotor and the camshaft, and is further oscillatable with respect to both the rotor and the camshaft between a fully retarded position and a filly advanced position. A locking configuration prevents relative motion between the rotor and the housing, and is mounted within either the rotor or the housing, and is respectively and releasably engageable with the other of either the rotor and the housing in the fully retarded position, the fully advanced position, and in positions therebetween. The locking device includes a locking piston having keys terminating one end thereof, and serrations mounted opposite the keys on the locking piston for interlocking the rotor to the housing. A controlling configuration controls oscillation of the rotor relative to the housing.
U.S. Pat. No. 6,374,787 shows a multi-position variable camshaft timing system actuated by engine oil pressure. A hub is secured to a camshaft for rotation synchronous with the camshaft, and a housing circumscribes the hub and is rotatable with the hub and the camshaft and is further oscillatable with respect to the hub and the camshaft within a predetermined angle of rotation. Driving vanes are radially disposed within the housing and cooperate with an external surface on the hub, while driven vanes are radially disposed in the hub and cooperate with an internal surface of the housing. A locking device, reactive to oil pressure, prevents relative motion between the housing and the hub. A controlling device controls the oscillation of the housing relative to the hub.
Referring to FIG. 1, a typical prior art feedback loop 10 is shown. The control objective of feedback loop 10 is to have a phaser disposed at a specified position, e.g. a spool valve in a null position by means of some type of actuator engaging a spool valve. In other words, the objective is to have no fluid flowing between two fluid holding chambers of a phaser (not shown) such that the VCT mechanism at the phase angle given by a set point 12 with the spool 14 stationary in its null position. This way, the VCT mechanism is at a desired phase position and the phase rate of change is zero. A control computer program product which utilizes the dynamic state of the VCT mechanism is used to accomplish the above state. The computer program product may either reside in the engine control unit (ECU), or it may reside somewhere independent of ECU.
The VCT closed-loop control mechanism is achieved by measuring a camshaft phase shift .θ0 16, and comparing the same to the desired set point r 12. The VCT mechanism is in turn adjusted so that the phaser achieves a position which is determined by the set point 12. A control law 18 compares the set point 12 to the phase shift θ0 16. The compared result is used as a reference to issue commands to an actuator such as solenoid 20 to position the spool 14. This positioning of spool 14 occurs when the phase error (the difference between set point 12 and phase shift 16) is non-zero.
The spool 14 is moved toward a first direction (e.g. right) if the phase error is positive (retard) and to a second direction (e.g. left) if the phase error is negative (advance). When the phase error is zero, the VCT phase equals the set point r 12. At this juncture, there is no need for adjustment as far as the feedback loop is concerned, so the spool 14 is held in the null position such that no fluid flows within the spool valve.
Camshaft and crankshaft measurement pulses in the VCT system are generated by camshaft and crankshaft pulse wheels 22 and 24, respectively. As the crankshaft (not shown) and camshaft (also not shown) rotate, wheels 22, 24 rotate along with them. The wheels 22, 24 possess teeth which can be sensed and measured by sensors according to measurement pulses generated by the sensors. The measurement pulses are detected by camshaft and crankshaft measurement pulse sensors 22a and 24a, respectively. The sensed pulses are used by a phase measurement device 26. A measurement phase difference is then determined. The phase difference is defined as the time from successive crank-to-cam pulses, divided by the time for an entire revolution and multiplied by 360.degree. The measured phase difference may be expressed as θ0 16. This phase difference is then supplied to the control law 18 for reaching the desired spool position.
A control law 18 of the closed-loop 10 is described in U.S. Pat. No. 5,184,578 and is hereby incorporate herein by reference. A simplified depiction of the control law is shown in FIG. 2. Measured phase 26 is subjected to the control law 18 initially at block 30 wherein a phase integration (PI) process occurs. Typically phase integration process is subdivided into two sub-processes. The first sub-process includes an amplification action; and the second sub-process includes an integration action. Measured phase is further subjected to phase compensation at block 32. Typically, a phase lag of the measured phase 26 is corrected therein.
When controlling the position of a Phaser in relation to a phase angle set point 12, the controlling spool valve 14 is at, or near its null position. To move the Phaser, the valve 14 is moved towards one end or the other end in proportion to the amount of error in the control loop. As shown supra, the error is the difference between the set point 12 and the phase angle 16 θ0 position feedback. When the Phaser is commanded to move to the mechanical stop or its mechanical limit, the control loop becomes ineffective because the position of the Phaser is dictated or limited by the positional stops and not by the error signal. For example, when the vane encounters a physical limit or stop in a housing but is still commanded to move toward the physically impossible direction, the integrator within the loop or the control law accumulates physically inaccurate hence undesirable information. In fact, when the Phaser is at its mechanical or physical stops, the error signal can cause the PID integrator 30 to try to move the spool valve 14. This causes the integrator 30 to keep increasing the error signal to try to move the Phaser. In order to stop this undesirable occurrence from happening, the usual method is to open the control loop and command the solenoid 20 to either full on or full off when it is within a few degrees of the positional stops. This approach may work well for a Phaser that uses oil pressure to move the Phaser but can cause noise for a Phaser that utilizes cam Torsionals to move the Phaser back and forth. Such torsional assisted phasers include the CTA patents listed supra such as the U.S. Pat. No. 5,657,725 commonly assigned to BorgWarner Inc, as well as single and dual check torsional assisted (TA) and the DM phasers.
Referring to FIG. 3, a prior art phaser 34 having a 4-way valve is shown. A pulse width modulated (PWM) 3 way valves (not shown) may also be used herein. Both of which are remotely mounted. A valve is remotely mounted in that the valve such as the spool valve 36 is not within the proximity of phaser chambers or rotor. A vane (not shown) divides a housing 38 into an advance chamber 40 and a retard chamber 42. A supply line 44 supplies pressurized fluid such as engine oil into the phaser chambers. Thereby the pressurized fluid in the phaser selectively causes the vane to move in one direction or the reverse of the direction according to a command. The result is that the fluid flowing into advance chamber 40 increasing the dimension thereto and the fluid flowing out of retard chamber 42 decreasing its dimension, or vice versa. The flowing of the fluid is enabled by an advance duct 46 and retard duct 48 working in conjunction. Both ducts 46, 48 possess a substantial length. Advance duct 46 has a first end connected to advance chamber 40 and a second end a second end connected to valve housing 50. Similarly, retard duct 48 has a first end connected to retard chamber 42 and a second end a second end connected to valve housing 50. Fluid flowing within both ducts can be controllably stopped by valve 36 which is engaged by an actuator 52. An outlet of the fluid is provided by an exhaust duct 54. It is noted that the exact position of the spool valve 36 in relation to the fluid ducts is not shown exactly. The control of the spool valve 36 can be any control mean described supra. A more detailed or exact depiction of the same is shown in FIGS. 4A, 4B, and 4C.
Referring to FIG. 4A, spool valve 36 at null position is shown. According to design requirements, at null position no fluid flows because spool valve stops the fluid from flowing by means of having both advance duct 46 and retard duct 48 blocked or sealed. At full advance position, as shown in FIG. 4B, spool 36 moves to a first position where supply fluid 44 is allowed to supply fluid via the spool valve 36, and ducts 46, 48 are permitted the unidirectional flow as shown. The unidirectional flow occurs because of the check valves or unidirectional valves (not shown). Exhaust duct 54a facilitates or completes the fluid circuit (only partially shown herein). At full retard position, as shown in FIG. 4C, spool 36 moves to a second position where supply fluid 44 is allowed to supply fluid, and ducts 46, 48 are permitted the unidirectional flow as shown because of the check valves or unidirectional valves (not shown). Exhaust duct 54b facilitates or completes the fluid circuit (only partially shown herein).
The graphs besides FIGS. 4A-4C indicate respectively the functional relationship between spool position (x-coordinate) and the flow rate (y-coordinate) into or out of chambers 40, 42. Further, note that spool 36 causes the ducts to be either completely upon or completely closed.
Referring now to FIG. 5, a known phaser is shown. Such phaser can be a phaser described in U.S. Pat. No. 5,107,804 by Thomas, J. Becker et al, commonly assigned to Borg-Warner Automotive & Engine Components Corporation. The phaser includes a housing 1 and a rotor 2. Housing 1 and rotor 2 are rotably coupled together. In other words, the phaser is interposed between two shafts. One of the shaft is a cam shaft 4 which, in this case, is rigidly attached to rotor 2. Housing 1, in turn, is rigidly attached to sprocket having a number of teeth 8 engaging a chain 9.
Referring again to FIGS. 3-5, advance duct 46 leads to advance chamber(s) and retard duct 48 leads to retard chamber(s). A pair of vanes 56 being an integral part of rotor 2 extends respectively into a pair of chamber region dividing the region into advance and retard chambers 40, 42 respectively. In a first advance chamber 40, a first wall 40a acts as a physical stop when a vane side wall 56a of vane 56 comes in direct physical contact with first wall 40a. Similarly, in a second advance chamber 40, a second wall 40b acts as a physical stop when a vane side wall 56b of vane 56 comes in direct physical contact with second wall 40b. Thereby, the rotable movement of vane 56 in relation to housing 1 is physically stopped when the direct physical contact occurs.
When spool valve 36 is commanded to advance or retard the vane 56, it may rotate toward the first wall 40a (see FIG. 4B) or rotate in the reverse direction (see FIG. 4C). When surfaces 56a, 56b are in the close proximity to, or in actual physical contact with, surfaces 40a, 40b respectively, noise occurs as a result of the physical contact between surfaces including surfaces 40a 56a and 40b 56b. 
When spool valve 36 is positioned at either fully advance or fully regard positions respectively, fluid is allowed to flow at its maximum rate (see graphs of FIGS. 4B and 4C. At full advance, the result is that full pressure is applied to keep phaser at full advance. Similarly, at full retard, full pressure is applied to keep phaser at full retard. For example, at full advance, surfaces 40a and 56a are at close proximity to each other or are in actual physical contact with each other. Typically, the force of the fluid flow is caused by torsional act of the cam shaft 4. As a result, noise occurs due to the oscillation of the vane at or near its physical stop at either end of its travel.
It is noted that similar surfaces exist in the pair of retard chamber 42. Typically, the physical components in the chamber area are symmetrical. Therefore, description of the same is omitted herein.
As can be appreciated, in a VCT system using torsionals to move a phaser back and forth, it is desirous to have a suitable device or process for eliminating noise at or in the proximity of the phase's physical stops or end of travel. Particularly, it is desirous to reduce noise in a TA and Dual Mode phaser used in an internal combustion engine.